Linear resistive temperature sensors (“RTD”) are today utilized in automotive control to monitor the high temperature in the exhaust pipes and in the catalyst of combustion engines.
The temperature to be monitored covers a wide range, from about −40° C. to about 1000° C., and the corresponding sensor resistance variation is from about 170Ω to about 850Ω, with quasi-linear temperature dependency. The resolution of the temperature measurement is therefore limited and measurement errors have a greater impact.
A conventional conditioning circuit for RTD sensors used in automotive controllers is shown in FIG. 1. This circuit consists of a very accurate pull-up resistor R1, particularly having a value of 1 k Ω, connected to an accurate supply voltage source Vcc, for example having a value of 5V. A sensor resistor RTD, which is a linear resistive temperature sensor, is connected in series between the pull-up resistor R1 and a voltage reference, particularly a ground conductor. A low pass filter 2 comprising a resistor Rf and a capacitor Cf is connected in parallel to the sensor resistor RTD, and is used to reduce the noise from the electrical environment. An analogue to digital converter ADC is connected in parallel to the filter 2 and is also connected to a reference voltage source VADC that tracks the supply voltage source Vcc. A microprocessor M is connected between the converter ADC and an output OUT of the circuit.
A voltage Vmeas across the sensor resistor RTD is measured at a node A with respect to ground, and applies:
                              V          meas                =                  RTD                                    R              1                        +            RTD                                              (        1        )            The resistance value of the sensor resistor RTD is obtained.
Specifically, the voltage Vmeas across the sensor resistor RTD is measured in a known manner and it is supplied to the converter ADC, which provides a digital value corresponding to said voltage. The digital value is supplied to the microprocessor M which calculates, according to equation 1, the resistance value of the sensor resistor RTD. Knowing the dependency between the resistance value of the sensor resistor RTD and the temperature, it is possible to obtain, at the output OUT of the circuit, the estimated value of the temperature.
The overall accuracy of the temperature measurement is mainly affected by: sensor resistance accuracy; conditioning circuit tolerances; quantization steps of the converter ADC; conversion errors of the converter ADC; leakage current of the converter ADC through the low pass filter 2.
The drawbacks of such architecture is that: it utilizes less than half span of the available converter input voltage range; the transfer function is non linear due to the voltage divider arrangement between the pull-up resistor R1 and the sensor resistor RTD; the sensitivity ΔVmeas/ΔTemperature is very low, for example not higher than about 1.2 mV/° C. at about @ 600° C.; the sensitivity ΔVmeas/ΔTemperature is not constant and decreases with the increase of the temperature; a very accurate and expensive pull-up resistor, particularly with 0.1% of tolerance, is required.
In view of the above, it is at least one object of the present invention to provide an alternative method for estimating the temperature in an internal combustion engine so as to improve the overall accuracy and sensitivity without the need to use complex circuits with expensive electronic components. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.
The at least one object, and other objects, desirable features, and characteristics, are achieved according to the present invention by the method. The method for estimating the temperature in an internal combustion engine comprising the steps of: providing a sensor resistor (RTD) in the internal combustion engine, the sensor resistor (RTD) having a predetermined resistance-temperature characteristic, measuring a sensor voltage (VRTD) across the sensor resistor (RTD), determining a resistance value of said sensor resistor (RTD) based on the sensor voltage (VRTD), and estimating the temperature based on said resistance value and the resistance-temperature characteristic. The method further comprises the steps of connecting, in series to the sensor resistor (RTD), a first branch of a current mirror arrangement, connecting a reference resistor (R0) in series to a second branch of the current mirror arrangement, measuring a reference voltage (V0) across the reference resistor (R0), and calculating the resistance value of the sensor resistor (RTD) based on the sensor voltage (VRTD)and the reference voltage (V0).
The at least one object, and other objects, desirable features, and characteristics, are achieved according to the present invention by a circuit. The circuit for estimating the temperature in an internal combustion engine comprises a sensor resistor (RTD) having a predetermined resistance-temperature characteristic, computing means (M) connected in parallel to the sensor resistor (RTD) and arranged to measure a sensor voltage (VRTD) across said sensor resistor (RTD), calculate a resistance value of the sensor resistor (RTD) based on the sensor voltage (VRTD), and estimate a temperature value using the resistance value and the resistance-temperature characteristic of the sensor resistor (RTD). The circuit being wherein it further comprises a first branch of a current mirror arrangement connected in series to the sensor resistor (RTD), and a reference resistor (R0) connected in series to a second branch of the current mirror arrangement.